Outlook 2007 + IMAP => e-mails messed up

Hello,

We are using OL 2007 to connect to our exchange2003 server. No pb.
Beside this, we have some special contact mailboxes, accessible with IMAP protocol, on a remote (external server).
These mboxes are shared by 3-4 users at a time.

It's been running fine for a while.
=> Now I can't open big e-mails (over 2 MB). When I do this, I got a popup telling me it's downloading the message, then  an error and I can't get my e-mail.
Furthermore, from this moment, outlook will start to mess up with my e-mails. When I click on an already downloaded e-mail, I might get the content of the next one... that kind of things.

I've tried these different things :
- deleted the imap account, recreate it (so a new .pst is created, maybe new settings) <- this worked on one computer but only for a few days.
- forced to download entire messages
- disabled auto receive/sending
- created a new profile, configuring the imap account in it to be sure to have a standard configuration
- tried it from different computers
- cleaned a lot of messages in the inbox, so the overall account has a reasonnable size.
==> same thing

On the other side :
- No error message on the remote (imap) server.
- the account from the same computer, with thunderbird, works just fine, even with big attachments

I must say our bandwidth is quite low (2 MB) and quite loaded. I've thought about timeouts, but again, at the same moment it's working in thunderbird...
Anyway, my webmail client to this IMAP account works fine and downloading attachments works, even if it's a bit long.
imho, outlook is a reaaal bad imap client (that's for sure, it barely knows there is a "sent" folder, and I'm not talking about the drafts or trash ones). Anyway, there might be something in outlook to help it manage an imap account a bit better.
I can't use thunderbird for various reasons (address book, end-users would have 2 mail clients and so on)

If you have any idea,

Thank you